package com.instacart.truetime.sntp;

/*
 * Original work Copyright (C) 2008 The Android Open Source Project
 * Modified work Copyright (C) 2016, Instacart
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import android.os.SystemClock;

import com.instacart.truetime.InvalidNtpServerResponseException;
import com.instacart.truetime.SntpEventListener;

import org.jetbrains.annotations.NotNull;

import java.io.IOException;
import java.net.DatagramPacket;
import java.net.DatagramSocket;
import java.net.InetAddress;

/**
 * Simple SNTP client class for retrieving network time. Original source: https://android.googlesource.com/platform/frameworks/base/+/master/core/java/android/net/SntpClient.java
 * <p>
 * Intentionally keeping this Java for easier diffing and keeping up to date with platform
 */
public class SntpImpl implements Sntp {

    public static final int RESPONSE_INDEX_ORIGINATE_TIME = 0;
    public static final int RESPONSE_INDEX_RECEIVE_TIME = 1;
    public static final int RESPONSE_INDEX_TRANSMIT_TIME = 2;
    public static final int RESPONSE_INDEX_RESPONSE_TIME = 3;
    public static final int RESPONSE_INDEX_ROOT_DELAY = 4;
    public static final int RESPONSE_INDEX_DISPERSION = 5;
    public static final int RESPONSE_INDEX_STRATUM = 6;
    public static final int RESPONSE_INDEX_RESPONSE_TICKS = 7;
    public static final int RESPONSE_INDEX_SIZE = 8;

    private static final int NTP_PORT = 123;
    private static final int NTP_MODE = 3;
    private static final int NTP_VERSION = 3;
    private static final int NTP_PACKET_SIZE = 48;

    private static final int INDEX_VERSION = 0;
    private static final int INDEX_ROOT_DELAY = 4;
    private static final int INDEX_ROOT_DISPERSION = 8;
    private static final int INDEX_ORIGINATE_TIME = 24;
    private static final int INDEX_RECEIVE_TIME = 32;
    private static final int INDEX_TRANSMIT_TIME = 40;

    // 70 years plus 17 leap days
    private static final long OFFSET_1900_TO_1970 = ((365L * 70L) + 17L) * 24L * 60L * 60L;

    public SntpImpl() {
    }

    /**
     * Sends an NTP request to the given host and processes the response.
     *
     * @param address host name of the server.
     */
    @NotNull
    @Override
    public synchronized SntpResult requestTime(
            InetAddress address,
            float rootDelayMax,
            float rootDispersionMax,
            int serverResponseDelayMax,
            int timeoutInMillis,
            SntpEventListener listener
    ) throws IOException {

        listener.sntpRequest(address);

        DatagramSocket socket = null;

        try {

            socket = new DatagramSocket();
            socket.setSoTimeout(timeoutInMillis);
            byte[] buffer = new byte[NTP_PACKET_SIZE];
            DatagramPacket request = new DatagramPacket(buffer, buffer.length, address, NTP_PORT);

            writeNtpVersion(buffer);

            // -----------------------------------------------------------------------------------
            // get current time and write it to the request packet

            long requestTime = System.currentTimeMillis();

            // TODO: Move android dependency to separate package
            //  so we can make Truetime a pure kotlin library
            long requestTicks = SystemClock.elapsedRealtime();

            writeTimeStamp(buffer, INDEX_TRANSMIT_TIME, requestTime);

            socket.send(request);

            // -----------------------------------------------------------------------------------
            // read the response

            long[] t = new long[RESPONSE_INDEX_SIZE];
            DatagramPacket response = new DatagramPacket(buffer, buffer.length);
            socket.receive(response);

            long responseTicks = SystemClock.elapsedRealtime();
            t[RESPONSE_INDEX_RESPONSE_TICKS] = responseTicks;

            // -----------------------------------------------------------------------------------
            // extract the results
            // See here for the algorithm used:
            // https://en.wikipedia.org/wiki/Network_Time_Protocol#Clock_synchronization_algorithm

            long originateTime = readTimeStamp(buffer, INDEX_ORIGINATE_TIME);     // T0
            long receiveTime = readTimeStamp(buffer, INDEX_RECEIVE_TIME);         // T1
            long transmitTime = readTimeStamp(buffer, INDEX_TRANSMIT_TIME);       // T2
            long responseTime = requestTime + (responseTicks - requestTicks);       // T3

            t[RESPONSE_INDEX_ORIGINATE_TIME] = originateTime;
            t[RESPONSE_INDEX_RECEIVE_TIME] = receiveTime;
            t[RESPONSE_INDEX_TRANSMIT_TIME] = transmitTime;
            t[RESPONSE_INDEX_RESPONSE_TIME] = responseTime;

            // -----------------------------------------------------------------------------------
            // check validity of response

            t[RESPONSE_INDEX_ROOT_DELAY] = read(buffer, INDEX_ROOT_DELAY);
            double rootDelay = doubleMillis(t[RESPONSE_INDEX_ROOT_DELAY]);
            if (rootDelay > rootDelayMax) {
                throw new InvalidNtpServerResponseException(
                        "Invalid response from NTP server. %s violation. %f [actual] > %f [expected]",
                        "root_delay",
                        (float) rootDelay,
                        rootDelayMax);
            }

            t[RESPONSE_INDEX_DISPERSION] = read(buffer, INDEX_ROOT_DISPERSION);
            double rootDispersion = doubleMillis(t[RESPONSE_INDEX_DISPERSION]);
            if (rootDispersion > rootDispersionMax) {
                throw new InvalidNtpServerResponseException(
                        "Invalid response from NTP server. %s violation. %f [actual] > %f [expected]",
                        "root_dispersion",
                        (float) rootDispersion,
                        rootDispersionMax);
            }

            final byte mode = (byte) (buffer[0] & 0x7);
            if (mode != 4 && mode != 5) {
                throw new InvalidNtpServerResponseException("untrusted mode value for TrueTime: " + mode);
            }

            final int stratum = buffer[1] & 0xff;
            t[RESPONSE_INDEX_STRATUM] = stratum;
            if (stratum < 1 || stratum > 15) {
                throw new InvalidNtpServerResponseException("untrusted stratum value for TrueTime: " + stratum);
            }

            final byte leap = (byte) ((buffer[0] >> 6) & 0x3);
            if (leap == 3) {
                throw new InvalidNtpServerResponseException("unsynchronized server responded for TrueTime");
            }

            double delay = Math.abs((responseTime - originateTime) - (transmitTime - receiveTime));
            if (delay >= serverResponseDelayMax) {
                throw new InvalidNtpServerResponseException(
                        "%s too large for comfort %f [actual] >= %f [expected]",
                        "server_response_delay",
                        (float) delay,
                        serverResponseDelayMax);
            }

            long timeElapsedSinceRequest = Math.abs(originateTime - System.currentTimeMillis());
            if (timeElapsedSinceRequest >= 10_000) {
                throw new InvalidNtpServerResponseException("Request was sent more than 10 seconds back " +
                        timeElapsedSinceRequest);
            }

            listener.sntpRequestSuccessful(address);

            return new SntpResult(t);

        } catch (Exception e) {
            listener.sntpRequestFailed(address, e);
            throw e;
        } finally {
            if (socket != null) {
                socket.close();
            }
        }
    }

    //region private helpers

    /**
     * Writes NTP version as defined in RFC-1305
     */
    private void writeNtpVersion(byte[] buffer) {
        // mode is in low 3 bits of first byte
        // version is in bits 3-5 of first byte
        buffer[INDEX_VERSION] = NTP_MODE | (NTP_VERSION << 3);
    }

    /**
     * Writes system time (milliseconds since January 1, 1970) as an NTP time stamp as defined in RFC-1305 at the given
     * offset in the buffer
     */
    private void writeTimeStamp(byte[] buffer, int offset, long time) {

        long seconds = time / 1000L;
        long milliseconds = time - seconds * 1000L;

        // consider offset for number of seconds
        // between Jan 1, 1900 (NTP epoch) and Jan 1, 1970 (Java epoch)
        seconds += OFFSET_1900_TO_1970;

        // write seconds in big endian format
        buffer[offset++] = (byte) (seconds >> 24);
        buffer[offset++] = (byte) (seconds >> 16);
        buffer[offset++] = (byte) (seconds >> 8);
        buffer[offset++] = (byte) (seconds >> 0);

        long fraction = milliseconds * 0x100000000L / 1000L;

        // write fraction in big endian format
        buffer[offset++] = (byte) (fraction >> 24);
        buffer[offset++] = (byte) (fraction >> 16);
        buffer[offset++] = (byte) (fraction >> 8);

        // low order bits should be random data
        buffer[offset++] = (byte) (Math.random() * 255.0);
    }

    /**
     * @param offset offset index in buffer to start reading from
     * @return NTP timestamp in Java epoch
     */
    private long readTimeStamp(byte[] buffer, int offset) {
        long seconds = read(buffer, offset);
        long fraction = read(buffer, offset + 4);

        return ((seconds - OFFSET_1900_TO_1970) * 1000) + ((fraction * 1000L) / 0x100000000L);
    }

    /**
     * Reads an unsigned 32 bit big endian number from the given offset in the buffer
     *
     * @return 4 bytes as a 32-bit long (unsigned big endian)
     */
    private long read(byte[] buffer, int offset) {
        byte b0 = buffer[offset];
        byte b1 = buffer[offset + 1];
        byte b2 = buffer[offset + 2];
        byte b3 = buffer[offset + 3];

        return ((long) ui(b0) << 24) +
                ((long) ui(b1) << 16) +
                ((long) ui(b2) << 8) +
                (long) ui(b3);
    }

    /***
     * Convert (signed) byte to an unsigned int
     *
     * Java only has signed types so we have to do
     * more work to get unsigned ops
     *
     * @param b input byte
     * @return unsigned int value of byte
     */
    private int ui(byte b) {
        return b & 0xFF;
    }

    /**
     * Used for root delay and dispersion
     * <p>
     * According to the NTP spec, they are in the NTP Short format viz. signed 16.16 fixed point
     *
     * @param fix signed fixed point number
     * @return as a double in milliseconds
     */
    private double doubleMillis(long fix) {
        return fix / 65.536D;
    }
    //endregion

}
